Hi Dave. This is a fairly common problem with heavy bags over time. There are two possible solutions. The first is just a redistributing of the bags inner contents by taking it down and working it sideways and upside down. Here is a step by step of that process - http://www.livestrong.com/article/482605-how-to-repack-a-punching-bag/

The other option is actually opening up the bag and removing or redistributing those sandbags which tend to sink to the bottom - http://www.youtube.com/watch?v=pffM1x2NwU4

When not using the bag, you could probably take it down and store it in an upside down position to reverse the natural effects of gravity.